Heat Index

How Heat Index Works

HI = −42.379 + 2.049T + 10.143R − 0.225TR − 0.00684T² − 0.0548R² + 0.00123T²R + 0.000853TR² − 0.00000199T²R²

T

Air Temperature — The actual air temperature in degrees Fahrenheit.

R

Relative Humidity — The relative humidity percentage (0-100).

HI

Heat Index — The apparent temperature or "feels like" temperature in °F.

How to Use

1. Enter the current air temperature in degrees Fahrenheit.
2. Enter the relative humidity percentage (0-100%).
3. The calculator applies the NOAA heat index formula to compute the feels-like temperature.
4. Review the danger tier and safety recommendations for your heat index value.

Understanding the Result

Frequently Asked Questions

Why does humidity make it feel hotter?

Your body cools itself primarily through the evaporation of sweat. When humidity is high, the air already contains significant moisture, which slows the evaporation rate. This impairs your body's primary cooling mechanism, making you feel hotter than the actual air temperature. At 100% humidity, sweat cannot evaporate at all.

Does the heat index account for direct sunlight?

No. The NOAA heat index assumes shady conditions with light wind. Direct sunlight can increase the apparent temperature by up to 15°F. On sunny days, it's recommended to add 10-15°F to the calculated heat index for an accurate feels-like temperature.

What is the difference between heat index and wet-bulb globe temperature?

The heat index considers only temperature and humidity. The Wet-Bulb Globe Temperature (WBGT) is a more comprehensive measure used by the military and athletic organizations that also accounts for solar radiation, wind speed, and cloud cover. WBGT is the gold standard for assessing heat stress during physical activity.

When should I call 911 for heat-related illness?

Call 911 immediately if someone shows signs of heatstroke: hot, red, dry skin (no sweating); body temperature above 103°F; rapid, strong pulse; confusion, altered mental state; dizziness; nausea; or loss of consciousness. Heatstroke is a medical emergency that can cause permanent organ damage or death.

How does humidity affect the heat index differently at night?

At night, the heat index can be just as dangerous as during the day, even though air temperatures are lower. When the sun goes down, the air cools, which increases relative humidity if the moisture content stays the same (cooler air holds less moisture, making the same amount of water vapor represent higher relative humidity). This means the feels-like temperature at night can remain high even as the actual temperature drops. For example, if the daytime high was 95°F with 50% humidity (heat index ~102°F) and the nighttime low is 80°F, the relative humidity typically rises to 80-90% because the same moisture content represents a higher percentage at the lower temperature. The heat index at 80°F with 90% humidity is about 86°F — still uncomfortable and potentially dangerous because the body cannot cool itself through sweat evaporation at high humidity. This is why heat advisories often remain in effect through the night during heat waves — the lack of nighttime cooling prevents the body from recovering, compounding heat stress over consecutive days. Urban heat island effects in cities can keep nighttime temperatures 5-10°F higher than surrounding rural areas, further increasing the risk.

Sources

• NOAA - Heat Index
• Wikipedia - Heat Index